Several fundamental aspects of the chemistry of dioxygen with biologically relevant transition metal ions will be explored. Emphasis is placed on reversible O2 binding, activation of O2 toward oxidation of organic substrates, and the production of O2 by oxidation of water. The approach taken here involves synthesis and characterization of small molecule analogs of the enzyme systems that perform these reactions. The proposed research is divided into the following projects: (1) synthesis and isolation of peroxide-bridged Fe and Mn complexes using pentadentate and hexadentate ligands; (2) preparation and characterization of single-atom- and peroxide bridged Fe, Cu binuclear complexes as models for the cytochrome c oxidase active site; (3) design and synthesis of sterically encumbered tri- and tetradentate ligands; (4) use of these ligands to prepare iron complexes of potential relevance to non-heme oxygenases and examination of their oxygen chemistry; (5) synthesis of peroxide-bridged dimanganese complexes with bulky ligands as models for reaction intermediates in photosystem II water oxidation chemistry, as well as further development of Mn-oxo chemistry.